Simple, the US has local, state and federal laws and depending on what is at issue local can trump state
or vice versa, state can trump federal or vice versa, the cases where federal trumps local or state are specific and defined.
Note that none of the federal level law enforcement arms are 'private police forces' all are public entities
and the reason they exist is the need to operate independently of local politics and concerns. Also some
things are of national concern and too big for local forces.
What you need to remember is that the US is large and diverse in terms of geography, population and politics and what works in rural Georgia won't necessarily work in urban Washington State.